Abstract

Remediation of contaminated soil, particularly flare pits, in oilfield sites has become a focus of increasingly stringent environmental legislation. These earthen pits consist of soil mixed with hydrocarbons, water, salts, and metals. Current remediation techniques such as biological or thermal treatment may have limited applicability in highly contaminated sites (e.g. land farming), or be excessively expensive (e.g. pyrolysis). This paper describes a cost-effective method for treating contaminated soil "in-situ" using low temperature oxidation (LTO) to convert the hydrocarbons to inert coke. The method was applied in a laboratory reactor to contaminated soil taken from a flare pit in Northwestern Alberta. The contaminated soil was oxidized with air at temperatures between 150 °C and 170 °C for 3 weeks. Extractable hydrocarbon levels were reduced to less than 0.1 % mass. Bioassays (14 day earthworm survival tests) showed that the toxicity associated with hydrocarbons was eliminated. Following LTO, remaining salts and metals were successfully removed by leaching with water. A conceptual design for application of LTO in the field is also presented in this paper.

Introduction

Contamination of soils by hydrocarbons, metals and salts is a hazard at many oil and gas well and processing sites. Unlined pits used for holding drilling fluids or flare pits are examples of these sources of contamination. Although such practices are no longer acceptable in the oil and gas industry, many of these contaminated pits exist, some dating back many years. Recently, the Government of Alberta through the Alberta Energy Utilities Board (AEUB) has issued new requirements for the remediation of such unlined earthen pits(1). Land-farming (i.e. the process of spreading contaminated waste on the surface of fields so that concentrations remain low, and gradually working it into the soil to promote biodegradation) is often the preferred remediation method because of its simplicity. However, many pits have high hydrocarbon and salt concentrations that preclude the feasibility of land-farming. Hence, there is a need to develop cost effective, alternative techniques for remediation of contaminated soil.

One such possible technique is the application of a Low Temperature Oxidation (LTO) process. This involves the injection of air or an oxygen-containing gas at moderate temperatures (150 – 200 °C) in the contaminated soil to convert the hydrocarbon contaminants into coke according to the following model:

Chemical equation (Available in full paper)

The dominant product, coke, is an inert, immobile and non-toxic substance consisting mostly of carbon. Low temperature oxidation of a wide variety of hydrocarbons (including aviation kerosene and bitumen) in the presence of core material and water has been investigated previously using plug flow reactors. For Athabasca bitumen, it was found that more than 80% of the initial oil was converted into coke after only a few hours of injecting oxygen/nitrogen mixtures with less than 10% oxygen content at temperatures ranging from 150 to 275 °C(2). These results suggest that the conversion to coke can be further augmented, and the temperature requirements reduced, by injecting gas with a higher oxygen content or by increasing the reaction time.

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